MODULE 1

Classification of the Senses

Philosophers have pointed out that we learn most of what we know about the
world through our sense organs. Everything we do depends first on the
information we receive through these senses. To what degree and under
what conditions this information is distorted depends on the very structure of
the senses themselves.

In this section, we will study human sensory capacities to learn what
factors set limits on our ability to receive environmental information, and to
find out just how many ways we have of obtaining this information.

As you read the text, try to answer the following questions.

What is a sensory system?

What are the four basic classes of senses according to the
classification based on anatomy?

What is Muller's doctrine of specific energy of nerves?

The senses can be classified in many ways. Aristotle distinguished five:
vision, hearing, smell, taste, and touch. We have learned a great deal since
Aristotle's time. In 1906, Sir Charles Sherrington suggested that the senses
might be classified in terms of the kind of information each sense organ
gives us. This type of ordering is called a functional classification.

The traditionalal functional classif cation of five senses
does not suit all types of inquiry

Sherrington distinguished four kinds of senses based on their functions.
Teleceptors give us information about our distant environment in much the
same way that a telescope brings distant information closer. Vision and
hearing fall into this category. Exteroceptors give us information about our
immediate external environment, that which we contact directly. Our skin
senses and sense of taste provide this type of information. Interoceptors
give us information about the bodily functions that make up our internal
environment,and proprioceptors give us information about bodily position.

Another classification system, the one we will be using, is based on
differences in anatomy. This system includes four basic classes of senses,
depending on where the end organs or receptors are located in the body.
These categories are special senses, skin senses, visceral senses and
deep senses. The special senses are those with specialized receptors or
end organs in the head. Our receptors for vision are the rods and cones of
the eye; we hear because of the action of hair cells in the ear. Smell and
taste are also special senses because they depend on specialized
receptors located in the nose and on the tongue. The vestibular sense,
which is the sense of balance or equilibrium, has its receptors in the
innermost part of the ear.

The anatomical classification
provides a more
complete description

Senses that have their receptors in the skin are called the skin senses.
They include touch, pain, and temperature.

Visceral senses give us information about our internal organs. For
example these senses tell us if the stomach is empty or the bladder full.

Deep senses include muscle, tendon, and joint sensations together with
deep pain and pressure.

What is a sensory system? In general a sensory system consists of an end
organ or group of receptor cells which respond to specific kinds of physical
energy. For example the eye responds to light. The receptors respond by
sending electrical impulses along the sensory nerves or nerves that lead
from the receptor cells in the eye to the brain.

The receptors, the sensory nerves, and those portions of the central
nervous system which process information for a particular sense make up
that sensory system.

Sensory information is constantly sent to the brain from receptors.
However, this is not the only direction in which impulses travel between a
receptor and the central nervous system. Neural paths called centrifugal
fibers lead from the brain to the sense organs. These centrifugal fibers
modify the response of the receptor to the physical stimuli. The brain is not
simply a passive recipient of anything the sense organs transmit. The brain
modifies information received from our environment by modifying the activity
of the sensory nerves or by modifying the sensitivity of the receptors. In so
doing, it modifies our perceptions of the world around us.

The specific energy theory
states that each receptor
conveys just one sensation

Our sensations also depend on the particular sensory system excited.
Johannes Muller arguing this point in 1840, stated that although the nerves
of each system are capable of responding to a number of different types of
stimuli, they are most sensitive to a particular kind of stimulus, and however
they are stimulated, their stimulation results in a specific sensation. For
example, if one were to press on his closed eye, he would get sensations of
"seeing."

Application of Muller's principle -- the doctrine of specific energy of nerves--
gives us what we call place theories the senses. We will encounter these
theories in our discussions of each of the senses.

Sensation results from physical stimulation of sensory systems

Our sensory systems are capable of making fine discriminations within the
range of physical stimuli we perceive. We are capable of distinguishing
between a tremendous number of sounds, and we can see the difference
between hundreds of hues. However, the actual range of physical stimuli to
which any sensory system is sensitive is well defined and measurable. For
example, compared to the possible range of wavelengths, the portion we
see as different colors of light is very small. If human eyes were constructed
so that we were sensitive to a different set of wavelengths of light, our world
would seem very different to us.

2. The system that classifies senses according to where the end organs or receptor cells are located in the body is based on:
a. individual senses.
b. anatomy.
c. function of the senses.
d. individual differences in sensitivity.

3. Muller's doctrine of the specific energy of nerves indicates that:
a. sensations depend on the particular sensory system excited.
b. each nerve of each system is capable only of responding to specific stimuli related
to that system.
c. (neither)

Information from the physical environment -- physical stimulli such as sound, smells, light, taste -- affect our
receptors (the eyes, ears, receptors in the nose and on the tongue). These receptors send information
along the nerve tracts to various structures in the brain. Different structures in the brain receive fibers from different nerves, and are therefore sensitive to different stimuli.

What are the components of a sensory system?____________________________________________2

One class of theories, the place theories, assumes that different
sensory systems are most sensitive to different stimuli.. These
theories stem from the doctrine of____________________________________________4

ANSWERS

2 receptors, nerve fibers, central

4 specific energeies

NOW TAKE PROGRESS CHECK 2

MODULE 1
PROGRESS CHECK 2

1. The four classes of senses based on differences in anatomy are:
a. vision senses, hearing senses, smell senses, and deep senses.
b. special senses, skin senses, visceral senses, and deep senses.
c. senses dealing with the distant environment, the immediate external environment, the internal environment, and those senses dealing with our body's spatial position in the environment.
d. (none of these)

2. Who developed the doctrine of specific energy of nerves?
a. Sherrington
b. Aristotle
c. Muller
d. (none of these)

3. Which of the following was not indicated by the doctrine of specific energy of nerves?

a. The sensory system that is excited determines the sensations we feel.
b. Each nerve of each system is capable of responding to many kinds of stimuli.
c. Place theories assume that different parts of the central nervous system are sensitive to
different stimuli.
d. (none of these)